Add more out of line copy ctors for complex classes.

cc/surfaces/surface_manager.cc

Index: cc/surfaces/surface_manager.cc
diff --git a/cc/surfaces/surface_manager.cc b/cc/surfaces/surface_manager.cc
index 23b49bc5df53fe1f52cdcf4c0aa7dfd9f7629492..d22d1e56c73f7892c8af708692caf0fb08c74522 100644
--- a/cc/surfaces/surface_manager.cc
+++ b/cc/surfaces/surface_manager.cc
@@ -17,6 +17,9 @@ namespace cc {
 SurfaceManager::ClientSourceMapping::ClientSourceMapping()
     : client(nullptr), source(nullptr) {}
 
+SurfaceManager::ClientSourceMapping::ClientSourceMapping(
+    const ClientSourceMapping& other) = default;
+
 SurfaceManager::ClientSourceMapping::~ClientSourceMapping() {
   DCHECK(is_empty()) << "client: " << client
                      << ", children: " << children.size();